Atrial fibrillation (AF) is a heart rhythm abnormality that involves irregular, and often rapid heartbeats. Billions of dollars are spent annually to diagnosis and treat this disorder. Recent studies demonstrate the feasibility of treating AF and other structural heart diseases with limited, left-atrial ablation lesion sets, using alternative energy sources These cardiac ablation procedures reduce the time required to perform the maze procedure surgery, and are less invasive. Radiofrequency (RF) remains the energy delivery system of choice, but it has several serious limitations, including the need for good electrode-tissue contact. This is difficult, and may lead to superficial lesions. Continuous wave lasers and laser diodes have been used experimentally, and while these laser treatments were superior, they required catheters that do not exist today, for producing long continuous transmural lesions. Intelligent Optical Systems (IOS) in conjunction with its collaborators, Dr. Hillel Laks of the UCLA Medical Center and Photonics Research Ontario, proposes to develop a unique fiber optic laser energy delivery catheter for the treatment of atrial fibrillation. This new instrument will provide physicians with a tool for producing viable lesions with minimum damage to the surrounding tissue. The proposed system will be a vast improvement over existing tools and will be valuable for the treatment of AF. Phase I research will focus on developing and testing a side emitting fiber optic diffuser at least 10cm long, fabricating and testing all critical elements of the catheter, and outlining the design of the working prototype. In Phase II, a working prototype of the catheter will be developed and tested in the laboratory, and in animal studies, at the UCLA Medical Center.